I don't know how I didn't see this thread until today. A couple of observations that may or may not be relevant. A 120R is a HD special order engine, correct?
If so, then normal engine # ID will not apply, it probably came with it's own MSO (manufacturers Statement of Origin).
Oil quantity: Should be the same as the engine it is replacing. In this case a Heritage (?). The 120 is larger due to bore & stroke, not case size, correct? So there's no more room in the oil bag than before.